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Pressure keeps tripping relief valve

thedra Member Posts: 2

I have a Burnham gas burner (232,000 btu [[sorry this was wrong, it's 180,000]]) that's about 25 years old. About 4 years ago I had the expansion tank (much older than boiler) replaced because apparently rust, etc had reduced the old tank's capacity and pressure would go above 30 psi tripping the relief valve (and releasing water). The plumber put in a bell & gossett expansion tank with a bladder and all was fine. Recently however, the pressure is getting back up to 30 psi and tripping the valve. The plumber came by yesterday and checked the system, including expansion tank, and all seemed fine, but replaced the relief valve just in case. Well, this morning, same thing happened, with water dribbling out of the relief valve. I should note that it seems to happen only in the morning when the house is reheating -- from being at 67 overnight to 71 in the morning. The boiler is in decent shape for its age, although the plumber cleaned a lot of soot, etc out yesterday. He thinks it has a few more years (caveat that it could break tomorrow or last 10 more years). What else could be causing the pressure? Thanks!


  • bustoff315
    bustoff315 Member Posts: 26

    There are a few things that could be causing the relief valve to pop. The pressure reducing valve may not be sealed tight and could be letting water slowly into the system. Where does the domestic water come from? Does it come from the boiler or is there a separate hot water heater or storage tank? If there is an internal coil there may be a small hole in that as well causing the pressure to rise.
  • Snowmelt
    Snowmelt Member Posts: 1,295
    2 words

    Pressure reducing valve

    Oops that's three words but seriously that is what I would look at,

    Also maybe an indirect water heater has a whole in it.
  • thedra
    thedra Member Posts: 2
    Domestic water?

    Hi not sure of your question. The leaking water comes from a pipe connected to the relief valve. The water to the boiler comes from the main line; there is a valve that regulates the amount of water that goes into the boiler. I have a separate water heater. Will check for leaks and tightness, thanks.
  • Jamie Hall
    Jamie Hall Member Posts: 17,065
    What bustof and Snowmelt

    are getting at, Thedra, is that your boiler is connected to your domestic water supply in at least one, and possibly two, ways.  One way is the boiler fill line -- the water supply for when, if ever, you have to refill the system.  That line is commonly connected to the system through a pressure reducing valve, the purpose of which is to reduce the domestic water pressure in your house to whatever the boiler is supposed to run at (ask your plumber -- could be around 15 to 20 psi).  That pressure reducing valve can, over time, decide to leak, and very very slowly allow the domestic water to push its way into the boiler, which will raise the pressure in the boiler.

    The quickest way to check that is to get the pressure in your boiler back down to where it is supposed to be, and then find the shutoff valve leading to that pressure reducing valve leading to the boiler -- and shut it off.  Then keep an eye on the boiler pressure.  It should hold pretty steady.  If it does, that pressure reducing valve is your problem.

    Now... it may be that you aren't set up that way, and that the only way to add water to your system should you ever need to is with a manual feed and shut off.  It's possible that that is leaking a little.

    A second way in which water can get into your boiler and mess things up is if your boiler also provides your domestic hot water.  If it does, there will be a coil in the boiler with domestic water in it.  If that coil develops a pinhole leak again you get domestic water in the boiler and the pressure goes up.  The way to check that, if you have that setup, is to isolate that domestic hot water coil and see if the pressure is steady.

    There is yet another possibility: that expansion tank may have become waterlogged or the diaphragm may have failed.  You would see that if the pressure went up when the system was hot, but dropped again when the system cooled off.  If that's what you are seeing, the expansion tank is your problem.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England